Near Field Communication is a wireless transmission technology that can transmit data at high speeds and over very short distances. The data exchange is performed by simply bringing close together two electronic devices, preferably a few centimeters; for example 10 centimeters or less. Near Field Communication is based on contactless Card and Radio Frequency Identification (RFID) technology.
Near Field Communication has a short read range typically limited to a range approximately equal to one wavelength. Actually, The Near Field signal decays as the cube of distance from the transmitter antenna.
A transmitter is a device that can read tags and write to them. This transmitter is also called a reader, because it can communicate with a tag through transmitting power and data to this tag, as well as through receiving data from this tag. To be able to communicate with passive tags, it generates an electromagnetic field from which the passive tag gets its energy. In a passive system, the transmitter transmits an energy that wakes up the tag and powers its chip, enabling it to transmit or store data.
A tag is a data carrier that can be read and possibly written using radio technology. Tags can comprise a battery or they can be passive. When tags are passive, they do not have their own power supply and are powered by the electromagnetic field of the transmitter. During the reading, the tag modulates the magnetic field and transmits the data by modulating the magnetic field.
An antenna is a conductive structure specifically designed to couple or radiate electromagnetic energy. Antenna structures, often encountered in NFC systems, may be used to both transmit and receive electromagnetic energy, particularly data-modulated electromagnetic energy. This method of communicating data between tags and readers in which information is carried through changes in the magnetic field created by the transmitter antenna uses inductive coupling between a transmitter antenna and a tag antenna.
In a communication process, the transmitter antenna transmits the electromagnetic energy to activate or awaken the tag, realizes the data transfer and sends the instructions to the tag. Meanwhile, the reader antenna receives information from the tag. The tag ability to efficiently extract energy from the transmitter field is based on the electrical resonance effect. Tag antenna element is designed to resonate at transmitter antenna operating frequency.
A transmitter communicates with a passive tag and powers it using the same signal. The fact that the same signal is used to transmit power and communicate data implies that any modulation of the signal causes a reduction in the power transmitted to the tag.
The powering of passive tags and the communication with those passive tags with the same communication signal places restrictions on the functionality and transactions the tags are capable of First, very little power is available to the digital portion of the integrated circuit on the tag, thus limiting the functionality of the tag. Second, the length of transactions with the tag is limited to the time for which the tag is expected to be powered and within communication range.
Actually, a big amount of the magnetic field generated by the transmitter is consumed by the simple entrance of a tag in the transmitter antenna field area. There is a drop in the magnetic field generated.
In a first prior art, for example described in the patent application U.S. Pat. No. 4,924,171, it is proposed to drive the transmitter generation of magnetic in accordance with the level of the signal of a tag. This drive takes place once the communication has been established between the tag and the reader. Active participation from tag through signal sending is needed for management of transmitter emitting power.
In a second prior art, for example described in the patent application WO2011067102, it is also introduced a contactless system in which the transmitter adapts the electromagnetic power level to the needs of a tag. In this document, there is a current associated to a level of magnetic field which is measured. The power extension request and information data from the tag are coded in different ranges. Here also, the process takes place once the communication has been established between the tag and the transmitter. Active participation from tag through message sending is needed for management of transmitter emitting power.
The issue of the management of the drop caused by the tag in the transmitter magnetic field is not tackled. This issue is all the more important as it is a requirement of the NFC standards. For example, the ISO14443 standard requires for the resulting magnetic field by the transmitter to range from 1.5 A/m to 7.5 A/m.
The strength of the field generated by the transmitter is classified into two kinds The loaded field corresponds to the field when a tag is present in the transmitter field. The un-loaded field corresponds to the field when no tag is present in the transmitter field.
Managing the drop is all the more critical as both modes have to be taken into account.
In general, with smaller antennas, there is a lower radiation resistance and a lower efficiency. Managing the drop is all the more challenging as there is a need to use smaller antennas in NFC systems for saving space reasons. This will help to obtain a size for transmitters and tags which is smaller and smaller.